Various studies of tire to surface friction and analysis of empirical data has shown that the maximum friction between a wheel and road surface occurs when the wheel is permitted to travel at a rotational velocity that is somewhat less than the relative velocity of the vehicle to the road surface. The difference between the wheel velocity and vehicle velocity is commonly referred to as the slip velocity. The slip ratio is a parameter that is derived by dividing the slip velocity by the vehicle velocity.
Antiskid and antilock brake control systems have been in widespread use for many years. In the simplest sense, an antiskid brake control system compares the speed of a vehicle derived from a wheel speed sensor (and wheel radius) to the vehicle speed typically derived from a secondary or reference source. If the wheel is determined to be skidding an excessive amount, then brake pressure applied to the wheel is released and the wheel is allowed to spin back up to the appropriate speed.
There are, of course, two major problems that become apparent in any such antiskid system. The first relates to determining the appropriate amount of skidding. The second relates to determining from where to obtain the reference velocity. The appropriate amount of skidding is described by the much discussed but seldom measured mu-slip curve. As is shown in FIG. 1, typically such curve is represented by the coefficient of friction .mu. between the wheel and the running surface on a vertical axis and the slip ratio on the horizontal axis. A slip ratio of zero is when the wheel is not skidding while a slip ratio equal to one represents a fully locked wheel.
In general, the slip ratio is increased by increasing the brake pressure in a braking system. It should be observed, however, that friction increases rapidly to a maximum, then begins to decrease with increasing slip ratio. This creates a control reversal problem as part of antiskid/antilock brake control when attempting to maintain maximum friction and provide optimal vehicular stopping distance. Furthermore, it has been found that lateral vehicle control is inversely proportional to the slip ratio. Consequently, it is preferable to maintain braking control at a location on the mu-slip curve which is slightly less than its peak. In order to avoid excessive wear on the wheel, typically the desired operating location on the mu-slip curve is slightly to the left of the peak of the mu-slip curve.
A problem that is often encountered when attempting to compute the slip velocity, and hence determining the appropriate amount of skidding, is excessive noise which is encountered on the wheel speed measurement. Such noise is caused, for example, by quantization effects from the wheel speed sensor, changes in the diameter of the wheel (both dynamic and wear related), or the movement of mechanical components attached to the wheel and speed sensor.
Regarding obtaining the vehicle/reference velocity in order to compute the slip ratio, it is often times necessary to rely on various accelerometers and other sensors to provide an accurate estimate of the reference velocity. This adds to the computational burden imposed on the brake control circuitry. Moreover, the cost associated with providing accurate and reliable sensors quickly adds up.
In view of the aforementioned shortcomings associated with existing antiskid/antilock brake control systems, there is a strong need in the art for a system which is substantially less susceptible to noise that may be encountered on the wheel speed measurement. In addition, there is a strong need for a system which does not require multiple sensors and which is able to operate based only on a single sensor that measures wheel speed. Moreover, there is a strong need for a system which does not place a large computational burden on the brake control system processor.